Linear relation between HI circular velocity and stellar velocity dispersion in early-type galaxies, and slope of the density profiles

TL;DR

This study finds a tight linear relation between HI circular velocity at large radii and stellar velocity dispersion in early-type galaxies, indicating a strong coupling between dark and luminous matter across galaxy regions.

Contribution

It establishes a novel link between outer HI kinematics and inner stellar dynamics in early-type galaxies using spatially resolved measurements, revealing insights into mass distribution.

Findings

v_circ(HI) = 1.33 σ_e with 12% scatter

v_circ(HI) is 25% lower than maximum v_circ from stellar models

Average density slope γ ≈ 2.18 with small scatter

Abstract

We report a tight linear relation between the HI circular velocity measured at 6 $R_{\rm e}$ and the stellar velocity dispersion measured within 1 $R_{\rm e}$ for a sample of 16 early-type galaxies with stellar mass between $10^{10}$ and $10^{11}$ $\mathrm{M}_\odot$. The key difference from previous studies is that we only use spatially resolved $v_\mathrm{circ}$(HI) measurements obtained at large radius for a sizeable sample of objects. We can therefore link a kinematical tracer of the gravitational potential in the dark-matter dominated outer regions of galaxies with one in the inner regions, where baryons control the distribution of mass. We find that $v_\mathrm{circ}$(HI) = 1.33 $\sigma_\mathrm{e}$ with an observed scatter of just 12 percent. This indicates a strong coupling between luminous and dark matter from the inner- to the outer regions of early-type galaxies, analogous to the situation in spirals and dwarf irregulars. The $v_\mathrm{circ}$(HI)-$\sigma_\mathrm{e}$ relation is shallower than those based on $v_\mathrm{circ}$ measurements obtained from stellar kinematics and modelling at smaller radius, implying that \vcirc\ declines with radius -- as in bulge-dominated spirals. Indeed, the value of $v_\mathrm{circ}$(HI) is typically 25 percent lower than the maximum $v_\mathrm{circ}$ derived at $\sim0.2\ R_\mathrm{e}$ from dynamical models. Under the assumption of power-law total density profiles $\rho \propto r^{-\gamma}$, our data imply an average logarithmic slope $\langle\gamma\rangle=2.18\pm0.03$ across the sample, with a scatter of 0.11 around this value. The average slope and scatter agree with recent results obtained from stellar kinematics alone for a different sample of early-type galaxies.